Effect of cooling rate upon the microstructure and mechanical properties of in-situ TiC reinforced high entropy alloy CoCrFeNi

doi:10.1016/j.jmst.2019.12.002

Abstract

Abstract:

Three types of in-situ TiC (5 vol%, 10 vol% and 15 vol%) reinforced high entropy alloy CoCrFeNi matrix composites were produced by vacuum induction smelting. The effect of two extreme cooling conditions (i.e., slow cooling in furnace and rapid cooling in copper crucible) upon the microstructure and mechanical properties was examined. In the case of slow cooling in the furnace, TiC was found to form mostly along the grain boundaries for the 5 vol% samples. With the increase of TiC reinforcements, fibrous TiC appeared and extended into the matrix, leading to an increase in hardness. The ultimate tensile strength of the composites shows a marked variation with increasing TiC content; that is, 425.6 MPa (matrix), 372.8 MPa (5 vol%), 550.4 MPa (10 vol%) and 334.3 MPa (15 vol%), while the elongation-to-failure (i.e., ductility) decreases. The fracture pattern was found to transit from the ductile to cleavage fracture, as the TiC content increased. When the samples cooled rapidly in copper crucible, the TiC particles formed both along the grain boundaries and within the grains. With the increase of TiC volume fraction, both the hardness and ultimate tensile strength of the resulting composites improved steadily while the elongation-to-failure declined. Therefore, the fast cooling can be used to drastically improve the strength of in-situ TiC reinforced CoCrFeNi. For example, for the 15 vol% TiC/CoCrFeNi composite cooled in the copper crucible, the hardness and ultimate tensile strength can reach as high as 595 HV and 941.7 MPa, respectively.

Key words: High entropy alloy matrix composites, Cooling rate, Microstructure, Mechanical properties

Jifeng Zhang, Ting Jia, Huan Qiu, Heguo Zhu, Zonghan Xie. Effect of cooling rate upon the microstructure and mechanical properties of in-situ TiC reinforced high entropy alloy CoCrFeNi[J]. J. Mater. Sci. Technol., 2020, 42: 122-129.

Figures/Tables 14

Table 1 Molar ratio of the composite specimens fabricated under different conditions.

Furnace cooled	Copper crucible cooled	TiC content (vol%)	Co	Cr	Fe	Ni	Ti	C
F₀	Cu₀	Matrix (0)	1	1	1	1	0	0
F₅	Cu₅	5	1	1	1	1	0.12	0.12
F₁₀	Cu₁₀	10	1	1	1	1	0.25	0.25
F₁₅	Cu₁₅	15	1	1	1	1	0.40	0.40

Fig. 1. Cooling rate curve of (a) furnace cooled and (b) copper crucible cooled samples.

Fig. 2. XRD patterns of CoCrFeNi alloy and its composites: (a) cooled in the furnace; (b) cooled in a copper crucible.

Fig. 3. Gibbs free energy change curves of Ni-Ti-C system.

Fig. 4. (a) SEM image of the Ni-Ti-C system, (b) energy dispersive spectrometer of TiC in the Ni-Ti-C system and (c) XRD patterns of the Ni-Ti-C system.

Fig. 5. SEM images of CoCrFeNi with different volume fractions of TiC particulates prepared from cooling in the furnace: (a) matrix; (b) 5 vol% TiC; (c) 10 vol% TiC; (d) 15 vol% TiC; (e) energy dispersive spectrometer of matrix; (f) energy dispersive spectrometer of TiC.

Fig. 6. SEM images of CoCrFeNi with different volume fractions of TiC prepared from cooling in a copper crucible after chemical etching: (a) matrix; (b) 5 vol% TiC; (c) 10 vol% TiC; (d) 15 vol% TiC.

Fig. 7. High magnification SEM images and elemental map analyses of CoCrFeNi+15 vol% TiC: (a) cooled in the furnace (sample F15); (b) cooled in a copper crucible (sample Cu15).

Fig. 8. TEM micrographs and diffraction patterns of different shape TiC in 15 vol% TiC/CoCrFeNi composites cooled in the furnace (sample F15): (a) micrograph of blocky TiC; (b) micrograph of chain shaped TiC; (c) diffraction pattern of blocky TiC; (d) diffraction pattern of chain shaped TiC.

Table 2 Mechanical properties of CoCrFeNi and its composites.

Volume fraction of TiC (vol%)	Vickers hardness (HV)		UTS (MPa)		Elongation (%)
Volume fraction of TiC (vol%)	Furnace cooled	Copper crucible cooled	Furnace cooled	Copper crucible cooled	Furnace cooled	Copper crucible cooled
0	288	291	425.6	498.0	70.9	70.1
5	391	412	372.8	811.2	18.9	51.7
10	446	546	550.4	916.0	8.3	30.0
15	513	595	334.3	941.7	3.7	18.1

Fig. 9. Vickers hardness of CoCrFeNi alloys load and its composites under 0.5 kg.

Fig. 10. Typical engineering stress-strain curves of CoCrFeNi and its composites: (a) cooling in the furnace; (b) cooling in a copper crucible.

Fig. 11. Fracture surfaces of the in-situ TiC/CoCrFeNi alloy composites with different volume fractions of TiC cooled in the furnace: (a) matrix; (b) 5 vol% TiC; (c) 10 vol% TiC; (d) 15 vol% TiC.

Fig. 12. Fracture surfaces of the in-situ TiC/CoCrFeNi alloy composites with various volume fraction of TiC cooling in a copper crucible: (a) matrix; (b) 5 vol% TiC; (c) 10 vol% TiC; (d) 15 vol% TiC.

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